High brightness paper coating applications demand high brightness pigments. Kaolin-based pigments are widely used in paper coating applications because of their desirable qualities, such as fiber coverage, paper gloss, and print gloss. High brightness kaolins are known in the art. For example, high brightness American, English, and Brazilian kaolins such as Capim DG have been used in paper coating applications. Despite advances that have led to improvements in kaolin pigment properties, however, prior art kaolin pigments lack the combination of brightness and particle size distribution and other properties required to further enhance high brightness paper coating applications. Consequently, some paper manufacturers use mixtures of kaolin and calcium carbonate pigments to coat or fill high brightness paper to achieve targeted high brightness levels.
Unfortunately, such mixtures increase the expense of high brightness paper. They also give rise to certain operational challenges. For instance, the pH at which paper coating machines operate must be kept neutral to alkaline to avoid degrading the calcium carbonate. Kaolins, however, require an acidic operating environment for the same reason. Changing the pH of a paper coating machine's operating conditions constitutes a major step with substantial costs, projected down-time, and risks in initial operating efficiency. Therefore, manufacturers of high brightness paper demand increasingly brighter kaolins that do not require calcium carbonate (or other pigment) additives to achieve target brightness levels. Such pigments must not only exhibit high brightness, but also preferably low titania or titania impurity levels, high shape factor, and other desirable kaolin qualities that render them useful in high speed paper coating applications. The present invention meets these and other needs.